JOLIET, Ill. — Facing the biggest credibility crisis in its long history, NASCAR issued a stern warning to its drivers and teams Saturday and said it won't tolerate any more attempts to alter the outcome of races.

After a scandal-filled week spent investigating teams and undoing attempts to manipulate its championship field, NASCAR came forward with a series of rules that will change the way teams have called races for years. NASCAR Chairman Brian France told teams he expects them “to give 100 percent” at all times, meeting with them for nearly 20 minutes at Chicagoland Speedway on the eve of the opening race of the Chase for the Sprint Cup championship.

“I think we wanted to be very clear and we wanted to reinforce the cornerstone of NASCAR, which is giving your all,” France said. “We addressed team rules, a variety of other things, all designed to do what our fans expect, and that means that their driver and their team give 100 percent to finish as high up in a given race as possible. We were very clear about that. That's our expectations.”

The warning came after an unprecedented week for NASCAR, which has been rocked by allegations of race-fixing since Clint Bowyer spun his car with seven laps remaining last Saturday night at Richmond, the race that completed the 12-driver field for the Chase. NASCAR was forced to investigate when it became clear that Bowyer spun in an attempt to stop leader Ryan Newman from winning and give teammate Martin Truex Jr. one last chance to earn a Chase berth. The investigation uncovered at least three instances of race manipulations and led to severe sanctions against Michael Waltrip Racing and the removal of unwitting participant Truex from the Chase in favor of Newman.

The central piece of evidence was radio communications, and the penalties against MWR set off a chain of events NASCAR never anticipated. Next came allegations of a scheme to sell track position and it triggered a new investigation involving deep-pocketed Penske Racing and tiny Front Row Motorsports. It culminated Friday with France's stunning decision to expand the Chase field to 13 drivers to accommodate Jeff Gordon, who had been bumped out of the Chase by the shenanigans of three drivers.